build: Omit -I... and -L... for standard paths in pkgconfig file

* configure.ac (GPGME_CONFIG_CFLAGS): Prefix with -I${includedir} for
non-standard include paths.
(GPGME_CONFIG_LIBS): Prefix with -L${libdir} for non-standard library
paths.
* src/gpgme.pc.in (Cflags): Remove hard-coded -I${includedir}.
(Libs): Remove hard-coded -L${libdir}.

This helps prevent problems when building/linking something that depends
on gpgme (installed in standard path) and some other library (installed
in a standard path and a custom path).

See T6136 for related changes in libgpg-error.